Photoactivated fluorescence from individual silver nanoclusters

Fluorescence microscopy of nanoscale silver oxide (Ag2O) reveals strong photoactivated emission for excitation wavelengths shorter than 520 nanometers. Although blinking and characteristic emission patterns demonstrate single-nanoparticle observation, large-scale dynamic color changes were also observed, even from the same nanoparticle. Identical behavior was observed in oxidized thin silver films that enable Ag2O particles to grow at high density from silver islands. Data were readily written to these films with blue excitation; stored data could be nondestructively read with the strong red fluorescence resulting from green (wavelengths longer than 520 nanometers) excitation. The individual luminescent species are thought to be silver nanoclusters that are photochemically generated from the oxide.     

Room-temperature ultraviolet nanowire nanolasers

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, <0001> oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.     

Free-electron lasers. Status and applications

A free-electron laser consists of an electron beam propagating through a periodic magnetic field. Today such lasers are used for research in materials science, chemical technology, biophysical science, medical applications, surface studies, and solid-state physics. Free-electron lasers with higher average power and shorter wavelengths are under development. Future applications range from industrial processing of materials to light sources for soft and hard x-rays.     

Diameter-Selective Raman Scattering from Vibrational Modes in Carbon Nanotubes

Single wall carbon nanotubes (SWNTs) that are found as close-packed arrays in crystalline ropes have been studied by using Raman scattering techniques with laser excitation wavelengths in the range from 514.5 to 1320 nanometers. Numerous Raman peaks were observed and identified with vibrational modes of armchair symmetry (n, n) SWNTs. The Raman spectra are in good agreement with lattice dynamics calculations based on C-C force constants used to fit the two-dimensional, experimental phonon dispersion of a single graphene sheet. Calculated intensities from a nonresonant, bond polarizability model optimized for sp2 carbon are also in qualitative agreement with the Raman data, although a resonant Raman scattering process is also taking place. This resonance results from the one-dimensional quantum confinement of the electrons in the nanotube.     

Phase-matched generation of coherent soft X-rays

Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.     

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at approximately 707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of approximately 8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.     

Deep-ultraviolet quantum interference metrology with ultrashort laser pulses

Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 x 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude.     

High-gain harmonic-generation free-electron laser

A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.     

Generation and photonic guidance of multi-octave optical-frequency combs

Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nanometers, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from a photonic band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kilowatt) infrared laser, producing 12-nanosecond-duration pulses. This represents a reduction by six orders of magnitude in the required laser powers over previous equivalent techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.     

Spontaneous ordering of oxide nanostructures

We report the spontaneous formation of uniformly distributed arrays of "tips" (tall conical hillocks) upon oxidation of palladium (Pd) thin films. The formation of the palladium oxide tips depended on the thickness and granularity of the Pd film and on annealing and oxidation conditions. As the Pd film thickness increased from 40 to 200 nanometers, the average height of the tips increased from 0.5 to 1.2 micrometers, their height distribution became broader, and their density decreased from 55 x 10(6) to 12 x 10(6) per square centimeter. Enhanced photoelectron emission from locations corresponding to the tips suggests their possible use in field emission applications.     

Ecosystem response to solar ultraviolet-B radiation: influence of trophic-level interactions

Solar ultraviolet radiation (UVR) can reduce the photosynthesis and growth of benthic diatom communities in shallow freshwater. Nevertheless, greater amounts of algae accumulate in UVR-exposed habitats than in UVR-protected environments. Near-ultraviolet (UVA wavelengths of 320 to 400 nanometers) and mid-ultraviolet (UVB, wavelengths of 280 to 320 nanometers) radiation also inhibit algal consumers (Diptera: Chironomidae). Larval chironomids are more sensitive to UVB than sympatric algae. Differential sensitivity to UVB between algae and herbivores contributes to counterintuitive increases in algae in habitats exposed to UVB. These mesocosm experiments illustrate that predictions of the response of entire ecosystems to elevated UVB cannot be made on single trophic-level assessments.     

Vision in annelid worms

In a first electrophysiological study of worm vision, electroretinograms were measured in two alciopid worms: Torrea, taken at the surface, and deep-sea Vanadis. Both forms possess a primary retina in the focal plane of the lens, and accessory retinas lying beside the lens. Such accessory retinas occur also in deep sea fishes and cephalopods. In Torrea the primary retina peaks in sensitivity at 400 nanometers, the secondary retina at 560 nanometers. Both together could serve as a depth guage, since 560 nanometers attenuates much faster in seawater than 400 nanometers. The Vanadis eyes peaked in sensitivity at 460 to 480 nanometers, a property shared with deep-sea forms of other phyla; and appropriate, since these wavelengths penetrate seawater most deeply, and also are the wavelengths of maximum bioluminescence.     

Microspectrophotometry of photoreceptor organelles from eyes of the prawn Palaemonetes

Microspectrophotometric measurements of individual dark-adapted rhabdoms of the prawn Palaemonetes vulgaris reveal the presence of two light-sensitive pigments. A pigment with maximum absorbancy at 555 nanometers is converted by light to a long-lived intermediate with wavelength of maximum absorbancy at 496 nanometers. A second pigment with wavelength of maximum absorbancy at 496 nanometers bleaches in the light, seemingly without forming detectable products at wavelengths longer than 375 nanometers. Both pigments occur in each layer of microvilli.     

Opponent color cells in the cat lateral geniculate nucleus

A microelectrode survey of the cat lateral geniculate has uncovered an infrequent new type of lateral geniculate cell in layer B with "on" center responses to short wavelengths and "off" center responses to long wavelengths. The short wavelength responses are mediated by cones with peak sensitivity at about 450 nanometers, and the long wavelength responses by cones with peak sensitivity at 556 nanometers. Two of double opponent color cells also had double opponent features.     

Evidence for large upward trends of ultraviolet-B radiation linked to ozone depletion

Spectral measurements of ultraviolet-B radiation made at Toronto since 1989 indicate that the intensity of light at wavelengths near 300 nanometers has increased by 35 percent per year in winter and 7 percent per year in summer. The wavelength dependence of these trends indicates that the increase is caused by the downward trend in total ozone that was measured at Toronto during the same period. The trend at wavelengths between 320 and 325 nanometers is essentially zero.     

Prolonged color blindness induced by intense spectral lights in rhesus monkeys

Prolonged exposure of rhesus monkeys to intense blue light produces long-term changes which are consistent with loss of response of those cones that contain a photopigment with peak absorption at 445 nanometers. The 90 percent reduction of spectral sensitivity in the blue region has lasted more than 5 months. Reduced sensitivity to long wavelengths is produced by adaptation to light of 520 nanometers. This reduced sensitivity, which lasts no more than 30 days, is attributed to a temporary loss of response of the cones containing a photopigment with peak absorption at 535 nanometers.     

X-ray pulses approaching the attosecond frontier

Single soft-x-ray pulses of approximately 90-electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/-1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach.     

Mutagenesis in Escherichia coli by visible light

Mutation to resistance to bacteriophage T5 in continuous cultures of Escherichia coli was induced by visible light (wavelength longer than 408 nanometers) and by black light (300 to 400 nanometers). Mutation rates more than 18 times greater than the spontaneous rate (no light) were obtained with moderate, nonlethal intensities of visible light. Mutation rates for both visible and black light were proportional to irradiance.     

Multichannel detection in high-performance liquid chromatography

A linear photodiode array has been used as the photodetector element in a new ultraviolet-visible detection system for high-performance liquid chromatography. This array allows simultaneous acquisition of light intensity data at all wavelengths between 190 and 600 nanometers. By use of a computer network concept in the electronics, this detection system can process eight different chromatographic signals simultaneously in real time and acquire spectra manually or automatically. Detector response times are variable and can be as low as 0.040 second, and bandwidth selection is variable from 4 to 400 nanometers. These characteristics permit fast chromatographic techniques and user-selectable signal-to-noise ratio enhancement. Spectra can be acquired in 10 milliseconds, permitting qualitative characterization at several different points on a single peak without destroying chromatographic signal integrity. Examples illustrate applications in fast high-performance liquid chromatography peak purity determination, and postanalysis data reduction.     

Spectroscopic observations of Mercury's surface reflectance during MESSENGER's first Mercury flyby

During MESSENGER's first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercury's surface.